Kidney Injury Molecule-1 (KIM-1) is a type 1 transmembrane protein that is not detectable in normal kidney tissue but is expressed at very high levels in dedifferentiated proximal tubule epithelial cells in human and rodent kidneys after ischemic or toxic injury. The extracellular domain of KIM-1 is cleaved and can be quantitated, by an ELISA assay, in the urine of rodents and patients with kidney injury or renal cell carcinoma. An important feature common to repair of the tissue after injury (e.g. acute kidney injury, AKI) or growth of cancer (e.g. renal cell carcinoma, RCC) is adequate supply of oxygen through formation of blood vessels. This led us to hypothesize that the function of KIM-1 ectdomain is to activate the endothelial cells by paracrine mechanisms and stimulate angiogenesis. In the preliminary results we found that KIM-1 ectodomain binds to endothelial cells and stimulates migration in a chemotactic and chemoattractant manner. Furthermore, KIM-1 also induced a 2-fold increase in the number of new blood vessels in the in vivo matrigel plug assay indicating its role in neovascularization. The objective of this proposal is to investigate the mechanisms involved in endothelial repair stimulated by KIM-1 following kidney toxicity due to drugs or environmental toxicants. In the first specific aim the expression of soluble KIM-1 will be characterized in relation with other biomarkers involved in endothelial repair process after kidney injury using various in vivo preclinical and clinical models of kidney toxicity. The second aim of this proposal is to investigate the mechanism of angiogenesis by KIM-1 in the context of endothelial cell regeneration by conducting structure function studies to identify the critical domain of KIM-1 responsible for endothelial cell survival, migration, tube formation and new blood vessel growth. We will also investigate if KIM-1 stimulates migration by inducing candidate gene expression further activating Rho and Integrin linked kinase pathways to cause chemotaxis. We will determine the critical role of KIM-1 in angiogenesis and endothelial resurtucturing by "Knock-in" and "Knockout" strategies using Kim-1 transgenic mice that overexpresses KIM-1 selectively on the epithelial cells of the embryonic and adult kidney proximal tubules and investigate if these mice have faster and more effective endothelial regeneration and recovery from kidney toxicity. Consequently, we will also use the KIM-1 knockout mice to determine if they are more susceptible to nephrotoxicity owing to the lack of repair. Since kidney is a major target for toxicity due to chemical annd physical agents, unraveling the mechanisms of how KIM-1 regulates growth of blood vessels would offer treatment paradigms to promote (in AKI) or inhibit (in kidney cancer) angiogenesis to ameliorate or perhaps even cure disorders that are leading causes of mortality today.